T cells are cells of the immune system. T cells express T cell receptors (TCRs) on their surface. The TCRs can recognize antigens and induce the T cell to generate an immune response against the recognized antigens. There are two main kinds of T cells namely, CD4 and CD8 T cells that typically generate an immune response. A small fraction of T cells, called regulatory T cells (Tregs), are typically involved in regulating the CD4 and CD8 T cell immune responses. Typically, CD4 T cells recognize antigens that are presented on antigen presenting cells in the context of an MHC Class II molecule. In contrast, CD8 T cells typically recognize antigens that are presented on antigen presenting cells in the context of an MHC Class I molecule.
Adaptive immunity to cancers and pathogens can be mediated by T cells, lymphocytes that are capable of identifying and killing cellular targets with exquisite specificity. This specificity is typically determined by the TCR, which is a heterodimer of two polypeptide chains. A typical wild-type (WT) TCR can be expressed on the surface of a T cell either as an alpha chain-beta chain heterodimer or as a gamma chain-delta chain heterodimer. During typical T cell development, the gene that encodes each polypeptide chain is uniquely rearranged by genetic recombination such that the sequence of each encoded polypeptide is unique for a given T cell. Each T cell typically expresses only one alpha and one beta, or one gamma and one delta chain.
Each TCR polypeptide chain comprises a variable domain, which confers specificity on the T cell, and several invariant domains including a constant domain, a connecting peptide, a transmembrane domain, and a short cytoplasmic tail. To achieve functional form, the TCR heterodimer typically recruits six additional chains which comprise the CD3 dimers gamma-epsilon, delta-epsilon and zeta-zeta. These additional chains can facilitate assembly and export of the TCR heterodimer to the T cell surface, as well as enable signal transduction upon engagement of a TCR heterodimer by its target antigen (Call et al., Ann. Rev. Immunol. 23: 101-125 (2005)).
Most T cells (˜90%) express TCRs that comprise an alpha chain and a beta chain. A smaller fraction (˜5-10%) of T cells express TCRs that comprise a gamma chain and a delta chain. Typically, a TCR heterodimer is assembled following recruitment of six additional polypeptide chains that form three CD3 dimers. The CD3 dimers are involved in expression of the heterodimeric TCR on the T cell surface. The CD3 dimers are also involved in signaling by TCRs following antigen recognition.
It has been observed that TCR heterodimers that are not properly assembled with the CD3 dimers prior to export are degraded (Bonifacino et al., J. Cell Biol. 109: 73-83 (1989)). Without being limited by any theory, the amino acid residues that are identified as involved in recruiting the CD3 dimers are present within the invariant domains of both chains of a TCR heterodimer (Call et al., Ann. Rev. Immunol. 23: 101-125 (2005); Kuhns et al., Immunity 26: 357-369 (2007); Xu et al., J. Biol. Chem. 281: 36977-36984 (2006)).